1. Field of the Invention
The invention pertains to an apparatus and method to simulate thermal loading of protective thermal barrier coatings to characterize thermal protection offered by such coatings that experience frequent, yet brief exposure periods of high-temperature and high-heat transfer conditions.
2. Description of the Prior Art
When large caliber guns are fired, their bores experience surface and subsurface damage caused by thermal effects, chemical attack by propellant gases, mechanical wear from projectile passage, and mechanical loading from gas pressurization. In particular, the internal surfaces of guns, (i.e. the bore, the face of the breech block and the primer vent if the gun uses a combustible cartridge case) suffer erosion from a number of sources. The passage of the hot gases from combustion of the primer and the main propellant at a high velocity causes considerable erosion of the primer vent, internal face of the breech block and the gun bores. The passage of the projectile along the barrel, its driving band engaging with the rifling grooves causes further erosion of the bore, which is enhanced by the escape of gases around the trailing edge of the driving band. The erosion results in the formation of pits in the bore and wearing away of accurately machined parts of the gun, such as the firing mechanism and rifling grooves. This is especially pronounced in sections that have been subjected to electrochemical corrosion resulting from deposition of pyrolysis products such as sulfides, nitrates and sulfates in small cracks in the gun from which they are not easily removed by cleaning. In addition, driving band debris builds up on the rifling grooves. This results in degradation of gun performance. The reclamation of gun components is a difficult and costly process, thus it is desirable to limit surface erosion if possible.
In particular, gun bore surfaces are typically subjected to short (5 to 10 millisecond) pulses of high thermal energy during firing of a munitions round. Included among the deleterious thermal effects resulting from exposure to these high thermal energy pulses are melting, phase transformations, and surface cracking (heat checking).
Although heating components and materials in a furnace can provide some information about the potential of a coating to protect a substrate, the slow, uniform heating typical of furnaces is quite different from the intense, yet brief, thermal conditions for which thermally protective coatings are most useful. For example, such a technique of using continuous accelerated heating with associated corrosion and erosion of the test specimen is taught in U.S. Pat. No. 5,793,042. Moreover, heating in a furnace does not solve the problem of duplicating the expected thermal environment and characterizing how the coating and substrate system behaves in transient-type environments, an essential condition of the test method of the present invention.
Known test apparatus that can simulate brief intense pulsed energy events include a xe2x80x9cvented combustorxe2x80x9d that can simulate, on a small scale, many of the actual conditions present in a large caliber gun during firing. In this device, a propellant charge is ignited in a confined space and at the appropriate time allowed to flow through an orifice, potentially exposing a test area to temperatures, pressures, and reaction products similar to those seen within a large caliber gun during firing. However, this method uses combustible, energetic agents and requires that utmost safety precautions be observed to prevent hazard and injury to personnel and test equipment before, during and after such test.
Although a vented combustor can come much closer than a furnace to reproducing the desired thermal conditions, this device is unsatisfactory in several ways. First, because of the complicated interaction of the several variables involved in using a vented combustor, it is difficult, and not necessarily possible, to ensure that it is adequately reproducing the desired heat transfer at the specimen surface. Second, using a vented combustor, one cannot isolate purely thermal effects from chemical and mechanical effects. Finally, and most critically, a vented combustor is grossly inconvenient and expensive device to operate. Special facilities and trained personnel are required for handling propellant and operating the device, making it a very expensive mode of testing.
The aforementioned prior art systems and methods do not afford the needed experimental controls for efficient, safe and time-accelerated testing of thermal protective barrier coatings, which the present invention resolves. Thus, the present invention provides a means for testing and evaluating capabilities of thermal barrier coatings on internal gun components that encounter brief high-energy conditions during gun firing.
The invention pertains to an apparatus and method for evaluating thermal barrier coating material on a surface of a part or component that is subjected to transient heat/thermal cycles. A pulsed laser heating apparatus that preferably includes lenses and optical fiber components produces conditions of brief heating on the test specimen.
In the method of the invention, the temporal shape, spatial distribution, and total energy of the laser pulse are designed to produce a spot of uniform illumination and heat absorption on the test specimen that closely approximates the thermal loading that the specimen is expected to receive during use. The test specimen is then examined for thermally induced changes.
Thermally protective barrier coatings are often useful when rapid and intense heating occurs at the surface of a component for brief periods of time (less than ten milliseconds, for example). In such applications, various coatings can reduce the peak temperature in the underlying material and thus extend its service life. Although the preferred use of the invention has been for testing TBC chromium coatings that are electrodeposited onto a high-strength steel alloy, where the application of such a coating is large caliber guns, the invention is nevertheless applicable to a broad range of other coatings and substrates, including ceramic and composite coatings where transient high-temperature conditions occur as well. Pulsed laser heating provides an easily controllable way to heat a coating and substrate system in a manner that can closely approximate the intense, yet brief, heating conditions for which thermally protective coatings are often useful.
Accordingly, advantages of the present invention include providing a testing laser heating apparatus and method that:
a) allows heat input to the test specimen and the duration of the heating pulse to be easily measured and easily controlled;
b) allows for pulsed laser heating that heats in a manner largely free from the interference of chemical and mechanical effects, thus isolating thermal effects from chemical and mechanical effects; and
c) effectuates material testing of transient heating on a test specimen that is much simpler, safer, and less expensive to operate.
Still further advantages will become apparent from consideration of the ensuing detailed description.